The present invention is directed to an electronic surface acoustic wave (SAW) component on, in particular, a lithium tantalate/niobate substrate that works with acoustic waves. Without this being respectively repeated, the invention can also be applied for potassium niobate and similar materials employed a substrate crystal for such components.
It is important for the employment and the operation of such a component, particularly for handsets of the mobile radiotelephone field, to preferably design the input/output filter (directly connected to the antenna) such that, on the one hand, it exhibits high power compatibility (for example, 2 Watts offered power) and, on the other hand, particularly has an extremely low insertion attenuation.
An applicable surface acoustic wave filter is disclosed, for example, by DE-A-2802946. It is essentially composed (see FIG. 7) of a piezoelectric substrate lamina 1 such as, for example, lithium tantalate on the selectively oriented substrate surface 2 of which electrode structures 12 such as inter-digital structures and the like are applied. LSAW waves (leaky surface acoustic waves) or HVPSAW(high velocity pseudo) waves proceed between these structures during operation (also see Ultrasonic Symp. Proc. 1994, pp. 281-286). These electrode structures and propagating waves 13 serve for the selective/filtering electrical signal transmission.
It is known for such a reflector/electrode structure (merely referred to overall below as electrode structures) having, for example, interdigital electrode fingers, reflector fingers and the like (also merely referred to overall as fingers below), that it is not only the sequence of the arrangement of the fingers 112 and their spacings that must be adhered to but that a predetermined dimension must also be adhered to (preferably within a range of tolerance) for the thickness (height) of the fingers, namely for adequately high reflectivity of these fingers.
For a high power compatibility, it is known from the Prior Art to provide special techniques for the electrode structures applied on the substrate surface. In general, these electrode structures are composed of, for example, photolithographically structured aluminum. Such structures of pure aluminum are relatively unstable in a number of ways. For enhancing the power compatibility, the aluminum has been alloyed with copper or a sandwich structure of aluminum and copper has been provided. What is thereby disadvantageous is that corrosion occurs given such a combination of materials. Adding titanium to the aluminum leads to higher electrical resistance of an electrode structure composed of this combination. Another approach that has been pursued is to apply the aluminum on the substrate with [11]-texture, a prior nucleation of the deposition surface thereof being required for this purpose. This not only causes higher technological expense, the reproducibility of such a textured aluminum layer leaves much to be desired. Epitaxial growth of the aluminum of an electrode structure is a distinctly expensive manufacturing method. A small grain structure of the aluminum for enhanced power compatibility can also be produced by sputtering the aluminum under correspondingly selected conditions. Disadvantageously, however, the photolithographic lift-off technique that is otherwise advantageous (and is preferably employed for the invention) can thereby not be applied for forming the electrode structure.
The desirable, low insertion attenuation for the component has also been mentioned above. It is known from the Prior Art (DE-A-19641662 and DE-C-2802946) to achieve low insertion attenuation of the surface acoustic wave filter in that a crystal of a section (red y) rotated by an angle xcex8 is employed as substrate lamina. The substrate lamina 1 has a surface 2 to which an axis system x1, x2, x3, with x2 as the normal N of this surface and with axes x1 and x3 lying in this surface are allocated. The known orientation of the crystallographic axis system x, y, z is such that the x-axis, coinciding with the axis x1, lies in the plane of the crystal section, i.e. in the substrate surface, and this crystallographic x-axis and the direction of the wave propagation 13 in the component are aligned parallel to one another. The y-axis of the crystal in the Prior Art resides on the substrate surface obliquely relative to the normal N thereof in the dimension of the rotational angle xcex8 corresponding to the rotation (red y). The z-axis therefore assumes the same angle xcex8 relative to the x1-x3 plane, i.e. to the substrate surface. The aforementioned publications specify an angular range between 38xc2x0 and 46xc2x0, on the one hand, and between 33xc2x0 through 39xc2x0, on the other hand for an angle xcex8 for the lithium tantalate. Angles with 66xc2x0 through 74xc2x0 and 41 xc2x0 are known for the lithium niobate.
An object of the present invention is to specify a concept of an applicable surface acoustic wave component (filter) that comprises the required power compatibility as presented above and that also preferably has minimized insertion attenuation.
According to the present invention, a surface acoustic wave component is provided comprising an electrode structure formed of fingers having aluminum as at least a principal material constituent on a surface with a surface normal of a pyroelectric and piezoelectric crystal substrate lamina of lithium tantalate. The surface is a crystal section such that the surface charges electrically positive given heating of the substrate lamina as a result of a pyroelectric effect. A crystal section is rotated around an x1=x-axis as a direction of wave propagation with an angle xcex94=(180xc2x0+xcex8) between a positive y-axis and the surface normal, wherein xcex8 is a known angle for low leakage wave losses for lithium tantalate crystal sections.
Also according to the invention, a surface acoustic wave component is provided having electrode structures formed of fingers of aluminum as at least a principal material constituent on a surface having a surface normal of a pyroelectric and piezoelectric crystal substrate lamina of lithium niobate. The surface is a crystal section such that said surface charges electrically positive given heating of the substrate lamina as a result of a pyroelectric effect. A crystal section is rotated around an x1=x-axis as a direction of the wave propagation with an angle xcex94=180xc2x0+xcex8 between a positive y-axis and the surface normal, wherein xcex8 is a known angle for low leakage wave losses for lithium niobate crystal sections.
Also according to the invention, a surface acoustic wave component is provided comprising electrode structures having fingers formed of aluminum as at least a principal material constituent on a surface having a surface normal of a pyroelectric and piezoelectric crystal substrate lamina of potassium niobate. The surface is a crystal section such that the surface charges electrically positive given heating of the substrate lamina as a result of the pyroelectric effect. A crystal section :is rotated around the x1=x-axis as a direction of wave propagation with an angle xcex94=180xc2x0+xcex8 between a positive.y-axis and the surface normal, wherein xcex8 is a known angle for low leakage wave losses for potassium niobate crystal sections.
The invention is described below on the basis of an example with lithium tantalate monocrystal substrate lamina (without being thereby considered limited thereto).
For achieving this object, a crystallographic orientation of the surface of the employed substrate lamina provided for the electrode structures 12 is selected that deviates significantly from the Prior Art. The +z-axis, which is also the pyroelectrical axis, resides at an angle xcex94 relative to the x1-x2 plane, i.e. it is directed into the substrate lamina relative to the substrate surface 11 (so that the negative z-axis points up away from the substrate surface in FIG. 1). The x3-axis is rotated counter-clockwise by the angle xcex94 relative to the +z-axis (as viewed opposite the x1-axis; also see FIG. 1). The same angle xcex94 is thus also present between the normal N (=x2-axis) and the +y-axis. Preferably, the angle xcex94=180xc2x0+xcex8, whereby xcex8 is an angle, in particular, between about 25xc2x0 and 46xc2x0.
For a respective crystal material such as lithium tantalate, lithium niobate, potassium niobate and the like, this angle xcex8 can be selected of the size that is standard in the Prior Art (as described above) for known surface acoustic wave components with a red y crystal section.
What this means in other words is that, compared to the Prior Art, the (parallel) back side of, for example, the respectively known section of the substrate lamina is employed here in the invention as the substrate surface for the electrode structures. This inventive technique that was previously not employed in the Prior Art is a surprisingly advantageous technique (as shall be presented later) in view of the stated object of enhancing the power compatibility of a surface acoustic wave component.
Lithium tantalate and the like is not only a piezoelectric but also a pyroelectric crystal. Due to the alignment of its surface normal N relative to the pyroelectric axis (equal to the Z-axis of the crystal) with the inventively selected angle xcex94=(180xc2x0+xcex8), the lamina surface inventively employed for the electrode structures/fingers charges to a positive voltage relative to the substrate lamina or, respectively, relative to its opposite, back surface of the lamina, namely when a heating of the substrate lamina or of at least its surface-proximate region under the electrode structures occurs.
Such a heating in fact occurs when a greater operational stressing of the surface acoustic wave component as an oscillator, filter or the like occurs. This process of heating and the positive charging connected therewith thus particularly and preferably occurs close to the electrode structures in that surface of the substrate lamina that, as inventively provided here (namely differing from the Prior Art), is selected for the application of the electrode structures/fingers.
The effect of actual interest that can be achieved with the invention is that, with the positive charging of the inventively selected surface, negative charges are drawn from the substrate lamina toward this surface. In particular, these are negatively charged oxygen ions. These act in situ on the aluminum of the electrode structures such that a stable aluminum oxide is formed in the influencing region of these oxygen ions. Advantageously for the invention, this aluminum oxide increases the power compatibility of the surface acoustic wave component, particularly the adhesion of the aluminum electrode structure on the inventively selected substrate surface. The self-heating of the component that occurs precisely given high electronic stressing of thereof thus inventively effects an intensified oxidation together with the accompanying advantage.
With the perception of this xe2x80x9cinternalxe2x80x9d action of the invention, the person skilled in the art is in the position to also apply the invention to other pyroelectric crystals that are not cited here, i.e. to apply the standard electrode structure of essentially aluminum on the selected substrate surface that charges positively given heating (occurring during operation) and to thus achieve the increased power compatibility in conformity with the object.
The exact size of the angle xcex94 (here, 180xc2x0+xcex8 with the above-recited dimensions or, respectively, limits for the angle xcex8) for the alignment of the pyroelectric axis of the (lithium tantalate and the like) crystal with respect to the surface of the substrate lamina employed for the electrode structures is relatively uncritical for this inventive effect of enhanced power compatibility insofar as this does not influence the operational sign of the above-described surface charging, i.e. the effect utilized for the invention. Greater heating due to increased dissipated power even leads to a more pronounced oxidation effect in the aluminum. Corresponding to the selection of the angle xcex8 in view of lower or, minimized leakage wave losses, however, it is nonetheless recommendable in the invention to select such an angular dimension for the angle part xcex8 (in xcex94=180xc2x0+xcex8), particularly the known angle values, with which low leakage wave losses are then also aspired to here.
Let the following description of the attached Figures belonging to the specification of the invention be provided for further explanation